Molecular Diversity and Associated Phenotypic Spectrum of Germline CBL Mutations

Noonan syndrome (NS) is a relatively common developmental disorder with a pleomorphic phenotype. Mutations causing NS alter genes encoding proteins involved in the RAS‐MAPK pathway. We and others identified Casitas B‐lineage lymphoma proto‐oncogene (CBL), which encodes an E3‐ubiquitin ligase acting as a tumor suppressor in myeloid malignancies, as a disease gene underlying a condition clinically related to NS. Here, we further explored the spectrum of germline CBL mutations and their associated phenotype. CBL mutation scanning performed on 349 affected subjects with features overlapping NS and no mutation in NS genes allowed the identification of five different variants with pathological significance. Among them, two splice‐site changes, one in‐frame deletion, and one missense mutation affected the RING domain and/or the adjacent linker region, overlapping cancer‐associated defects. A novel nonsense mutation generating a v‐Cbl‐like protein able to enhance signal flow through RAS was also identified. Genotype–phenotype correlation analysis performed on available records indicated that germline CBL mutations cause a variable phenotype characterized by a relatively high frequency of neurological features, predisposition to juvenile myelomonocytic leukemia, and low prevalence of cardiac defects, reduced growth, and cryptorchidism. Finally, we excluded a major contribution of two additional members of the CBL family, CBLB and CBLC, to NS and related disorders.     

Mutation spectrum in the nephrin gene (NPHS1) in congenital nephrotic syndrome

Congenital nephrotic syndrome, Finnish type (CNF or NPHS1), is an autosomal recessive disease characterized by massive proteinuria and development of nephrotic syndrome shortly after birth. The disease is most common in Finland, but many patients have been identified in other populations. The disease is caused by mutations in the gene for nephrin which is a key component of the glomerual ultrafilter, the podocyte slit diaphragm. A total of 30 mutations have been reported in the nephrin gene in patients with congenital nephrotic syndrome worldwide. In the Finnish population, two main mutations have been found. These two nonsense mutations account for over 94% of all mutations in Finland. Most mutations found in non-Finnish patients are missense mutations, but they include also nonsense and splice site mutations, as well as deletions and insertions. This mutation update summarizes the nature of all previously reported nephrin mutations and, additionally, describes 20 novel mutations recently identified in our laboratory.     

Mutations in COQ8B (ADCK4) found in patients with steroid‐resistant nephrotic syndrome alter COQ8B function

Mutations in COQ8B cause steroid‐resistant nephrotic syndrome with variable neurological involvement. In yeast, COQ8 encodes a protein required for coenzyme Q (CoQ) biosynthesis, whose precise role is not clear. Humans harbor two paralog genes: COQ8A and COQ8B (previously termed ADCK3 and ADCK4). We have found that COQ8B is a mitochondrial matrix protein peripherally associated with the inner membrane. COQ8B can complement a ΔCOQ8 yeast strain when its mitochondrial targeting sequence (MTS) is replaced by a yeast MTS. This model was employed to validate COQ8B mutations, and to establish genotype–phenotype correlations. All mutations affected respiratory growth, but there was no correlation between mutation type and the severity of the phenotype. In fact, contrary to the case of COQ2, where residual CoQ biosynthesis correlates with clinical severity, patients harboring hypomorphic COQ8B alleles did not display a different phenotype compared with those with null mutations. These data also suggest that the system is redundant, and that other proteins (probably COQ8A) may partially compensate for the absence of COQ8B. Finally, a COQ8B polymorphism, present in 50% of the European population (NM_024876.3:c.521A > G, p.His174Arg), affects stability of the protein and could represent a risk factor for secondary CoQ deficiencies or for other complex traits.     

EIF2S3 Mutations Associated with Severe X‐Linked Intellectual Disability Syndrome MEHMO

Impairment of translation initiation and its regulation within the integrated stress response (ISR) and related unfolded‐protein response has been identified as a cause of several multisystemic syndromes. Here, we link MEHMO syndrome, whose genetic etiology was unknown, to this group of disorders. MEHMO is a rare X‐linked syndrome characterized by profound intellectual disability, epilepsy, hypogonadism and hypogenitalism, microcephaly, and obesity. We have identified a C‐terminal frameshift mutation (Ile465Serfs) in the EIF2S3 gene in three families with MEHMO syndrome and a novel maternally inherited missense EIF2S3 variant (c.324T>A; p.Ser108Arg) in another male patient with less severe clinical symptoms. The EIF2S3 gene encodes the γ subunit of eukaryotic translation initiation factor 2 (eIF2), crucial for initiation of protein synthesis and regulation of the ISR. Studies in patient fibroblasts confirm increased ISR activation due to the Ile465Serfs mutation and functional assays in yeast demonstrate that the Ile465Serfs mutation impairs eIF2γ function to a greater extent than tested missense mutations, consistent with the more severe clinical phenotype of the Ile465Serfs male mutation carriers. Thus, we propose that more severe EIF2S3 mutations cause the full MEHMO phenotype, while less deleterious mutations cause a milder form of the syndrome with only a subset of the symptoms.     

Mutation Update for Kabuki Syndrome Genes KMT2D and KDM6A and Further Delineation of X‐Linked Kabuki Syndrome Subtype 2

Kabuki syndrome (KS) is a rare but recognizable condition that consists of a characteristic face, short stature, various organ malformations, and a variable degree of intellectual disability. Mutations in KMT2D have been identified as the main cause for KS, whereas mutations in KDM6A are a much less frequent cause. Here, we report a mutation screening in a case series of 347 unpublished patients, in which we identified 12 novel KDM6A mutations (KS type 2) and 208 mutations in KMT2D (KS type 1), 132 of them novel. Two of the KDM6A mutations were maternally inherited and nine were shown to be de novo. We give an up‐to‐date overview of all published mutations for the two KS genes and point out possible mutation hot spots and strategies for molecular genetic testing. We also report the clinical details for 11 patients with KS type 2, summarize the published clinical information, specifically with a focus on the less well‐defined X‐linked KS type 2, and comment on phenotype–genotype correlations as well as sex‐specific phenotypic differences. Finally, we also discuss a possible role of KDM6A in Kabuki‐like Turner syndrome and report a mutation screening of KDM6C (UTY) in male KS patients.     

A Clinical and Molecular Analysis of Branchio‐Oculo‐Facial Syndrome Patients in Russia Revealed New Mutations in TFAP2A

Branchio‐oculo‐facial syndrome (BOFS, OMIM# 113620) is a rare autosomal dominant disorder characterised by branchial cleft sinus defects, ocular anomalies and facial dysmorphisms, including lip or palate cleft or pseudocleft, and is associated with mutations in the TFAP2A gene. Here, we performed clinical analysis and mutation diagnostics in seven BOFS patients in Russia. The phenotypic presentation of BOFS observed in three patients showed high heterogeneity, including variation in its main clinical manifestations (linear loci of cervical cutaneous aplasia, ocular anomalies and orofacial cleft). In certain other cases, isolated ocular anomalies, or an orofacial cleft with accessory BOFS symptoms, were observed. In five BOFS patients, conductive hearing loss was diagnosed. Direct sequencing of the coding region of the TFAP2A gene revealed missense mutations in four BOFS patients. One patient was observed to have a previously described mutation (p.Arg251Gly), while three patients from two families were found to have novel mutations: p.Arg213Ser and p.Val210Asp. These novel mutations were not present in healthy members of the same family and therefore should be classified as de novo.     

Mutations in the human laminin β2 (LAMB2) gene and the associated phenotypic spectruma

Mutations of LAMB2 typically cause autosomal recessive Pierson syndrome, a disorder characterized by congenital nephrotic syndrome, ocular and neurologic abnormalities, but may occasionally be associated with milder or oligosymptomatic disease variants. LAMB2 encodes the basement membrane protein laminin β2, which is incorporated in specific heterotrimeric laminin isoforms and has an expression pattern corresponding to the pattern of organ manifestations in Pierson syndrome. Herein we review all previously reported and several novel LAMB2 mutations in relation to the associated phenotype in patients from 39 unrelated families. The majority of disease‐causing LAMB2 mutations are truncating, consistent with the hypothesis that loss of laminin β2 function is the molecular basis of Pierson syndrome. Although truncating mutations are distributed across the entire gene, missense mutations are clearly clustered in the N‐terminal LN domain, which is important for intermolecular interactions. There is an association of missense mutations and small in frame deletions with a higher mean age at onset of renal disease and with absence of neurologic abnormalities, thus suggesting that at least some of these may represent hypomorphic alleles. Nevertheless, genotype alone does not appear to explain the full range of clinical variability, and therefore hitherto unidentified modifiers are likely to exist. Hum Mutat 31:992–1002, 2010. © 2010 Wiley‐Liss, Inc.     

Phenotypic overlap of McKusick‐Kaufman syndrome with Bardet‐Biedl syndrome: A literature review

Hydrometrocolpos (HMC) and post‐axial polydactyly (PAP) are common to both McKusick‐Kaufman syndrome (MKS) and Bardet‐Biedl syndrome (BBS). We review reported cases of MKS and BBS presenting with HMC and PAP early in life to determine if there are clinical features that allow discrimination between the two syndromes as the primary features of retinitis pigmentosa, obesity, learning disability in BBS are age‐dependent. We did not find any phenotypic features that allowed reliable differentiation between the two syndromes in the neonatal period. However, uterine, ovarian, and fallopian tube anomalies are more common in BBS patients, and it may be that these clinical features prove to be useful discriminating features. We conclude that sporadic female infants with HMC and PAP cannot be diagnosed with MKS until at least age 5 years and that monitoring for the complications of BBS should be performed in these patients. Am. J. Med. Genet. 95:208–215, 2000. Published 2000 Wiley‐Liss, Inc.     

KCNH2 Gene Mutation: A Potential Link Between Epilepsy and Long QT-2 Syndrome

Long QT syndrome (LQTS) is closely associated with syncope, seizure, and sudden death but LQTS is frequently misdiagnosed as epilepsy. LQTS and epilepsy both belong to the group of ion channelopathies that manifest in the heart and brain. Therefore, genetic analysis of genes associated with potassium and sodium homeostasis and electrical disorders may reveal a link between epilepsy and lethal cardiac arrhythmia. Here, the authors report a young woman who suffered recurrent seizure episodes and syncopes that occurred while walking and also during rest. She showed electroencephalogram abnormalities and a pathological prolonged QTc interval in electrocardiogram. The patient and the patient's asymptomatic family members underwent genetic screening of the three genes most frequently associated with LQTS: KCNQ1, KCNH2, and SCN5A. The patient and the family members did not show DNA alterations in the genes KCNQ1 and SCN5A associated with LQT-1 and LQT-3, respectively. However, the patient showed a de novo mutation 2587T→C in exon 10 of KCNH2 gene associated with LQT-2. The mutation caused a stop codon substitution (R863X) in the HERG channel, leading to a 296–amino acid deletion. The patient's asymptomatic relatives did not show the KCNH2 gene mutation. R863X alteration in HERG channel may be involved in both prolonged QTc interval and epilepsy. This fact raises the possibility that R863X alteration in KCNH2-encoded potassium channel may confer susceptibility for epilepsy and cardiac LQT-2 arrhythmia.     

Whole‐Exome Sequencing Identifies a Variant of the Mitochondrial MT‐ND1 Gene Associated with Epileptic Encephalopathy: West Syndrome Evolving to Lennox–Gastaut Syndrome

We describe a West syndrome (WS) patient with unidentified etiology that evolved to Lennox–Gastaut syndrome. The mitochondrial respiratory chain of the patient showed a simple complex I deficiency in fibroblasts. Whole‐exome sequencing (WES) uncovered two heterozygous mutations in NDUFV2 gene that were reassigned to a pseudogene. With the WES data, it was possible to obtain whole mitochondrial DNA sequencing and to identify a heteroplasmic variant in the MT‐ND1 (MTND1) gene (m.3946G>A, p.E214K). The expression of the gene in patient fibroblasts was not affected but the protein level was significantly reduced, suggesting that protein stability was affected by this mutation. The lower protein level also affected assembly of complex I and supercomplexes (I/III₂/IV and I/III₂), leading to complex I deficiency. While ATP levels at steady state under stress conditions were not affected, the amount of ROS produced by complex I was significantly increased.     

肾小管标志物测定在原发性肾病综合征治疗前后的意义

目的:通过测定肾小管标志物判断原发性肾病综合征(PNS)病人的肾小管受损程度,以期探讨在不同疗效的病人治疗前后的意义.方法:应用放射免疫分析测定白介素-6(IL-6)、内皮素-1(ET-1)、α1-微球蛋白(α1-m)、β2-微球蛋白(β2-m);用ELISA测定纤维蛋白原降解产物(FDP);速率法测定N-乙酰β-D氨...     

RASopathy‐Associated CBL Germline Mutations Cause Aberrant Ubiquitylation and Trafficking of EGFR

Noonan syndrome, a congenital disorder comprising a characteristic face, short stature, heart defects, learning difficulties, and a predisposition to malignancies, is caused by heterozygous germline mutations in genes encoding components of RAS‐MAPK signaling pathways. Mutations in the CBL tumor suppressor gene have been reported in patients with a Noonan syndrome‐like phenotype. CBL encodes a multivalent adaptor protein with ubiquitin ligase activity, which promotes ubiquitylation and vesicle‐mediated internalization and degradation of the epidermal growth factor (EGF) receptor (EGFR). We investigated the functional consequences of disease‐associated CBL amino acid changes p.K382E, p.D390Y, and p.R420Q on ligand‐induced EGFR trafficking. Expression of CBL^K382E, CBL^D390Y, or CBL^R420Q in COS‐7 cells resulted in increased levels of surface EGFR and reduced amounts of intracellular EGFR; both consequences indicate ineffective EGFR internalization. Accordingly, receptor‐mediated uptake of EGF was decreased. Furthermore, the p.K382E, p.D390Y, and p.R420Q lesions impaired CBL‐mediated EGFR ubiquitylation and degradation. Together, these data indicate that pathogenic CBL mutations severely affect vesicle‐based EGFR trafficking. Since we detected enhanced ERK phosphorylation in cells expressing mutant CBL, we conclude that aberrant EGFR trafficking contributes to augmented RAS‐MAPK signaling, the common trait of Noonan syndrome and related RASopathies. Thus, our data suggest that EGFR trafficking is a novel disease‐relevant regulatory level in the RASopathy network.     

Base‐Biased Evolution of Disease‐Associated Mutations in the Human Genome

Understanding the evolution of disease‐associated mutations is fundamental to analyze pathogenetics of diseases. Mutation, recombination (by GC‐biased gene conversion, gBGC), and selection have been known to shape the evolution of disease‐associated mutations, but how these evolutionary forces work together is still an open question. In this study, we analyzed several human large‐scale datasets (1000 Genomes, ESP6500, ExAC and ClinVar), and found that base‐biased mutagenesis generates more GC→AT than AT→GC mutations, while gBGC promotes the fixation of AT→GC mutations to balance the impact of base‐biased mutation on genome. Due to this effect of gBGC, purifying selection removes more deleterious AT→GC mutations than GC→AT from population, but many high‐frequency (fixed and nearly fixed) deleterious AT→GC mutations are remained possibly due to high genetic load. As a special subset, disease‐associated mutations follow this evolutionary rule, in which disease‐associated GC→AT mutations are more enriched in rare mutations compared with AT→GC, while disease‐associated AT→GC are more enriched in mutations with high frequency. Thus, we presented a base‐biased evolutionary framework that explains the base‐biased generation and accumulation of disease‐associated mutations in human populations.     

Role of Immunofluorescence and Electron Microscopy in the Evaluation of Renal Biopsies in Nephrotic Syndrome in a Developing Country

ABSTRACT

To determine the role of immunofluorescence (IF) and electron microscopy (EM) in the evaluation of renal biopsies in a developing country, the authors carried out a study in 200 patients with nephrotic syndrome. Renal biopsies were studied by light microscopy, IF, and EM. IF study was useful in all, being essential in 23.5% and helpful in remaining cases. EM was useful in 94.5% cases, being essential in 43% and helpful in 51.5% cases. The results demonstrate that IF and EM are essential in the evaluation of renal biopsies in nephrotic syndrome and these should be employed in the pathologic evaluation of renal biopsies.     

New Mutations Associated with Rasopathies in a Central European Population and Genotype–Phenotype Correlations

We performed the genetic analysis of Rasopathy syndromes in patients from Central European by direct sequencing followed by next generation sequencing of genes associated with Rasopathies. All 51 patients harboured the typical features of Rasopathy syndromes. Thirty‐five mutations were identified in the examined patients (22 in PTPN11, two in SOS1, one in RIT1, one in SHOC2, two in HRAS, three in BRAF, two in MAP2K1 and two in the NF1 gene). Two of them (p.Gly392Glu in the BRAF gene and p.Gln164Lys in the MAP2K1 gene) were novel with a potentially pathogenic effect on the structure of these proteins. Statistically significant differences in the presence of pulmonary stenosis (63.64% vs. 23.81%, P = 0.013897) and cryptorchidism (76.47% vs. 30%, P = 0.040224) were identified as the result of comparison of the prevalence of phenotypic features in patients with the phenotype of Noonan syndrome and mutation in the PTPN11 gene, with the prevalence of the same features in patients without PTPN11 mutation. Cryptorchidism as a statistically significant feature in our patients with PTPN11 mutation was not reported as significant in other European countries (Germany, Italy and Greece). The majority of mutations were clustered in exons 3 (45.45%), 8 (22.73%), and 13 (22.73%) of the PTPN11 gene.     

Mutation Spectrum and Genotype–Phenotype Correlation in Cornelia de Lange Syndrome

Cornelia de Lange syndrome (CdLS) is a clinically and genetically heterogeneous developmental disorder. Clinical features include growth retardation, intellectual disability, limb defects, typical facial dysmorphism, and other systemic involvement. The increased understanding of the genetic basis of CdLS has led to diagnostic improvement and expansion of the phenotype. Mutations in five genes (NIPBL, SMC1A, SMC3, RAD21, and HDAC8), all regulators or structural components of cohesin, have been identified. Approximately 60% of CdLS cases are due to NIPBL mutations, 5% caused by mutations in SMC1A, RAD21, and HDAC8 and one proband was found to carry a mutation in SMC3. To date, 311 CdLS‐causing mutations are known including missense, nonsense, small deletions and insertions, splice site mutations, and genomic rearrangements. Phenotypic variability is seen both intra‐ and intergenically. This article reviews the spectrum of CdLS mutations with a particular emphasis on their correlation to the clinical phenotype.